Underground drilling involves drilling a bore through a formation deep in the Earth using a drill bit connected to a drill string. During rotary drilling, the torque applied at a top drive of a drilling rig is often out of phase with the rotational movement at the bottom-hole assembly (BHA) of the drill string due to an elasticity of the material of the drill string. This causes the drill string to yield somewhat under the opposing loads imposed by the rotational force at the top drive and friction/inertia at the end where the bit is located (e.g., the BHA). This causes resonant motion to occur between the top drive and the BHA that is undesirable. Further, as the drill string winds up along its length due to the ends being out of phase, the torque stored in the winding may exceed the static friction force when the bit is sticking, causing the drill string near the bit to slip relative to the wellbore sides at a high (and often damaging) speed.
“Stick-slip” is a downhole condition where torsional vibrations have increased because the bit and BHA are experiencing increased friction and drag at the bit, causing the bit to stop rotating. Once the bit has stopped rotating, torque tends to build up in the drill string. The torque buildup causes the energy in the drill string to increase until it overcomes the drag friction between the bit/BHA and the formation, which frees the bit momentarily until the drag friction overcomes the rotational energy in the drill string again. This causes a periodic motion called stick-slip.
Stick-slip is a major contributing factor to excessive bit wear, as well as damage to motors, downhole tools, and the BHA. Torsional vibration can have the effect that cutters on the drill bit may momentarily stop or be rotating backwards, i.e., in the reverse rotational direction to the normal forward direction of rotation of the drill bit during drilling. This is followed by a period of forward rotation of many times the RPM mean value.
Measured torque of the drill string may be used in addition to other techniques to adjust a rotation speed during the rotary drilling to reduce the chance of stick-slip and/or other vibrations. In an approach, impedance between the top drive and the drill string (i.e., any torsional waves traveling up the drill string) is sought to be matched by analyzing RPM feedback from an encoder of a motor (e.g., a motor in a top drive or rotary table). As a result, the drive (e.g., a variable-frequency drive) is detuned to achieve as near to a constant torque as possible, resulting in changes to RPM of the top drive. Another approach is more active in changing speed to match impedance between the top drive and the drill string. The RPM value for the top drive is adjusted based on the feedback obtained from torque. Matching impedance by itself, however, will not eliminate all stick-slip occurrences. In addition, these approaches can result in significant swings in top drive RPM speed, creating concern of damage to the drill string and the top drive when the swing is particularly large.
Thus, what are needed are systems, apparatuses, and methods that provide more effective ways to reduce stick-slip conditions from happening in the first place.